[terminal] Support an API to notify the command execution is done in terminal

Let's make sure issue is not already fixed in latest builds first.

- [x] I verified [latest Integration Build of Eclipse SDK](https://download.eclipse.org/eclipse/downloads/) doesn't provide the feature.

## Suggestion

I hope to collect the output of a command execution in the terminal, thus I need a signal to let my plugin knows the execution is done.

## Community

- [x] I understand suggesting an enhancement doesn't mandate anyone to implement it. Other contributors may consider this suggestion, or not, at their own convenience. The most efficient way to get it fixed is that I implement it myself and contribute it back as a good quality patch to the project.